In recent years, a variety of portable electronic devices have been becoming widespread owing to rapid development of techniques for downsizing electronic devices. Batteries as power sources for these portable electronic devices are also required to be downsized, and nonaqueous electrolyte secondary batteries having a high energy density receive attention.
Particularly, attempts have been made to use materials having a high lithium absorption capacity and a high density, such as elements that form an alloy with lithium, such as silicon and tin, and amorphous chalcogen compounds. Among them, silicon is capable of absorbing lithium at a ratio of 4.4 lithium atoms per silicon atom, and its negative electrode capacity per mass is about 10 times of that of graphite carbon. However, silicon has a problem in cycle life such that a change in volume associated with insertion and desorption of lithium in the charge-discharge cycle is significant, leading to size reduction of active material particles.
The present inventors have extensively conducted experiments, and resultantly found that an active material formed by combining and firing minute silicon monoxide and a carbonaceous material, wherein microcrystalline Si is dispersed in the carbonaceous material while being encompassed or held in SiO2 that is strongly bound with Si, is obtained, so that capacity enhancement and improvement of cycle characteristics can be achieved. However, even with such an active material, the capacity is reduced when several hundred charge-discharge cycles are performed, and thus life characteristics are not sufficient for long-term use.
Further, the process of a reduction in capacity has been minutely examined, and resultantly it has been found that microcrystalline Si contained in the active material is grown while charge-discharge is repeated, so that the crystallite size increases. There is the problem that influences of a change in volume associated with insertion and desorption of Li during charge-discharge become significant due to the growth of the crystallite size, leading to a reduction in capacity.